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Introduction to Linux - A Hands on Guide

This guide was created as an overview of the Linux Operating System, geared toward new users as an exploration tour and getting started guide, with exercises at the end of each chapter.
For more advanced trainees it can be a desktop reference, and a collection of the base knowledge needed to proceed with system and network administration. This book contains many real life examples derived from the author's experience as a Linux system and network administrator, trainer and consultant. They hope these examples will help you to get a better understanding of the Linux system and that you feel encouraged to try out things on your own.

I was getting annoyed at the authentication request on installing new software - looked at sudoers and had NOPASSWD : ALL on admin and sudo so I stupidly removed the % at the start of each line.

I have tried entering via recovery mode as root to fix this problem but when editing via sudo visudo it states file system is read only and if I attempt to su root with my password I just get authentication failure.

bit stuck on what to try next and would dearly love to avoid booting from disc (mainly because I can't find my usb and don't want to wipe my install completely).

If you can login as root, you can fix the problem by directly modifying the /etc/sudoers file. To get root login, you can always reboot to single-user or recovery mode.

visudo is not necessary in single-user mode. It is a wrapper around a normal editor (defined in EDITOR environment variable, defaults to some editor, usually vi), which makes sure the file is edited atomically; that all other processes see either the old contents, or the new contents, but never anything that combines the two. It is used to avoid race conditions and editing mishaps, but is in no way required.

Therefore, if you can login as root, you can then edit the sudoers file using your favourite editor, for example using nano (nano /etc/sudoers) or vi (vi /etc/sudoers).

However using nano /etc/sudoers when I attempt to save edits it says file system is read-only!

Ah, that just means your recovery mode mounts the root filesystem read-only. It is a good thing; it means no changes are done (can be done!) to the filesystem, unless you explicitly allow it. (If one of your disks crashes, it is better that the system does not attempt to do any fixes, or do any changes at all to the disk contents, before you have had the opportunity to safely copy (or try to copy) your files to some other storage media.)

Fortunately, you only need to run one command to switch from read-only file system to a normal one: